The eli15 answer is that concrete is repaired by excising the damage and replacing what was removed. If you're really interested check out Sikas brochures.0 if you prefer video form I found this [1] that seems to cover a good variety of applications and can probably be comfortably watched at 2x speed.

Maintaining concrete is a matter of having routine structural inspectors come out to poke and prod the concrete. Worn traffic areas get patches and sealants. Areas with a lot of runoff get seals to direct water in ideal ways. Etc.

0. https://www.sika.com/en/construction/structural-strengthenin...

1. https://youtu.be/c0Pj5Qdy9IU

Pretty much. Water is the enemy of any construction.

A farm near where I grew up had a big barn, built in the 1700s that was in daily use until the mid 80s. Due to a family/inheritance dispute, maintenance ceased in the 90s. At some point, the roof leaked and that set in motion things that resulted in collapse last year.

Probably the most important one: keeping the roof from leaking. Uncontrolled moisture incursion is one of the quickest ways to destroy any structure.

plugging/checking for holes in the roof so there's no water infiltration in the walls (water in the wall will quickly degrade them, especially if temperatures to freezing levels)